This issue does appear to be a false positive problem introduced by a ClamAV signature update as best we can tell. I have tested with the updated signatures (this morning) and I am no longer seeing the same failures, which is in line with William's comment above. To test this in your own environment, head over to Settings>Antivirus and use the menu to update the ClamAV Virus Definitions. At that point you should be good to enable the ClamAV scanning again. Please monitor for any further issues as well.
Kyle Kerst
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
www.smartertools.com